Computational Chemistry

6.3.4 Properties Arising from Electron Distribution: Dipole

Moments, Charges, Bond Orders

The discussion in Section5.5.4on dipole moments, charges and bond orders applies
in a general way to the calculation of these quantities by semiempirical methods
too. Electrostatic potentials, whether visualized as regions of space or mapped onto
van der Waals surfaces, are usually qualitatively the same for AM1 and PM3 as for
ab initio methods. Atoms-in-molecules calculations are not viable for semiempiri-
cal methods, because the core orbitals, lacking in these methods, are important for
AIM calculations.

80

60

40

20

0

IR_INTENS

0

43

87

1.3e+002

4000

4000

3000

3500 3000

2000

2500 2000

1000

1500 1000 500

Benzene^3091

Experimental^3050

AM1

1579

3195

744

1040

674

1480

IR_INTENS

0

37

73

1.1e+002

4000 3500 3000 2500 2000 1500 1000 500

MP2 / 6-31G*

3239 1540

689

1083

FREQ_VAL

Fig. 6.6 Experimental (gas phase), AM1 and ab initio (MP2(fc)/6–31G*) calculated IR spectra of
benzene

426 6 Semiempirical Calculations